Primary infection with varicella-zoster virus (VZV) causes chickenpox, and reactivation of the virus from latency results in zoster. We have previously identified a cellular protein, insulin-degrading enzyme (IDE), that interacts with a VZV glycoprotein, gE and functions as a receptor for entry of the virus into cells. VZV gE interacts with glycoprotein I and the two proteins form a complex on the surface of the virus and on virus-infected cells. Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for virus infectivity and binds to a cellular receptor, insulin-degrading enzyme (IDE), through its unique amino terminal extracellular domain. Previous work showed that IDE plays an important role in VZV infection and virus cell-to-cell spread, which is the sole route for VZV spread in vitro. This year we found that recombinant soluble IDE (rIDE) enhanced VZV infectivity at an early step of infection associated with an increase in virus internalization, and increased cell-to-cell spread. rIDE also enhanced the stability of cell-free virus over a wide range of temperatures. A VZV mutant lacking the IDE binding domain of gE was impaired for syncytia formation and membrane fusion. This mutant also accumulated on the cell surface and at cell-cell junctions. Pre-treatment of cell-free VZV with rIDE markedly enhanced the stability of the virus over a range of conditions. rIDE interacted with gE to elicit a conformational change in gE and rendered it more susceptible to proteolysis. Co-incubation of rIDE with gE modified the size of gE. These data suggest that the conformational change in gE elicited by IDE enhances infectivity and stability of the virus and leads to increased fusogenicity during VZV infection. The varicella and zoster vaccines used in the United States must be stored at -20oC or colder, and used within 30 minutes after they are reconstituted. The ability of rIDE to enhance infectivity of cell-free VZV over a wide range of incubation times and temperatures suggests that rIDE may be useful as an additive for the cell-free varicella and zoster vaccines to improve their stability. Using the Oka vaccine virus as a backbone, this year we constructed a VZV mutant which has a small deletion in both copies of VZV gene 63 and insertion of two copies of a cassette containing (a) truncated version of VZV gene 62, (b) VZV gene 63 with a small deletion, and (c) full-length VZV gene 64 between VZV genes 64/65 and 68/69. The virus was not impaired for growth in human cells, induced higher levels of neutralizing antibodies in guinea pigs, and was impaired for latency in rodents compared with the Oka vaccine virus (p=0.0022). Additional mutants containing the same truncation in gene 62, with or without the gene 63 deletion, were less impaired for latency. A VZV Oka mutant, replicating to similar titers and inducing a comparable immune response as the Oka vaccine virus, but impaired for latency, might serve as a safer vaccine and be less likely to reactivate to cause zoster.